This invention relates to the joining of aluminum to metal surfaces by brazing and, more particularly, to a totally convertible brazing encapsulate or laminate for use in brazing aluminum to surfaces such as aluminum, copper, brass surfaces.
It is common practice to join aluminum components by disposing an aluminum brazing alloy between or adjacent the component surfaces to he joined, and heating the brazing alloy and the joining surfaces in appropriately assembled fashion to a temperature (brazing temperature) at which the brazing alloy melts while the components remain unmelted. Upon subsequent cooling the brazing alloy forms a filet or joint that bonds the joining surfaces of the components. For assured selective melting of only the brazing alloy in heating step, it is commonly preferred that the melting point of the brazing alloy be at least 30.degree. to 40.degree. C. lower than that of the metal of the components. An example of a typical aluminum brazing alloy is an aluminum-silicon eutectic composition, which starts to melt at about 577.degree. C.
Chemical Abstract 193273 83-450066 refers to a Lucas-Milhaupt publication entitled "Designing for Preforms--Gold/Silver/Aluminum/Soft Solder/Copper". It describes flux-filled solder preforms, which may he used for brazing. There is no disclosure of a brazing laminate or encapsulate that is totally convertible to a eutectic alloy during brazing.
Kiser, U.S. Pat. No. 4,994,640 issued Feb. 19, 1991, discloses a welding electrode whose core is filled with flux. The electrode comprises a sheath made up of Ni and Mn, while the flux contains Mn, Si, Al, Ca and Mg. This electrode is used in welding, not brazing to make a weld pool of Ni and Mn. An eutectic alloy is not formed.
Zvanut et al, U.S. Pat. No. 4,214,145, issued Jul. 22, 1980, describes an electrode comprising a sheath of metal surrounding a hollow core filled with iron powder, along with manganese and silicon. This electrode is used in arc welding, not in brazing, and no eutectic alloy is generated when the electrode is heated.
Yutaka, U.S. Pat. No. 4,831,701, issued May 23, 1989, discloses a composition for brazing zinc metal or a zinc alloy to the surface of an aluminum workpiece. The metal is deposited by vacuum vapor deposition or by ion plating the metal onto the surface of a potassium fluoroaluminate flux. The flux is encapsulated by a metal or alloy and there is a reaction between the metal and the substrate upon heating. However, there is no reaction between the encapsulating metal and the material encapsulated to produce an alloy and the material that is encapsulated is an inorganic salt, a potassium fluoraluminate.
In U.S. Pat. No. 3,971,501 (Cooke) there is described a method of joining aluminum surfaces with an aluminum brazing alloy in which a mixture of the aluminum brazing alloy in powdered form and a powdered flux is coated on the surfaces to be joined. The coating is then dried and the surfaces are then heated to a brazing temperature whereby the brazing is completed. This procedure, like other known systems, utilizes a brazing alloy.
Although fluxless brazing procedures have been devised, their use is limited because of economic and other considerations arising from the special conditions and equipment required for successful practice of such procedures. It is, therefore, generally necessary to employ a flux in brazing aluminum, to remove the oxide ordinarily present on exposed metal surfaces at the location of the joint, as well as to promote flow of molten brazing alloy during the heating step. The material used as a flux must be capable of dissolving and/or otherwise removing the metal oxides at the brazing temperatures while remaining essentially inert with respect to the aluminum at such temperatures. Since fluxes are usually reactive only when at least partially molten, fluxes for aluminum brazing should as a practical matter be partly or wholly molten at brazing temperatures, e.g. at temperatures not substantially higher and preferably lower than 577.degree. C.
It is the object of the present invention to provide a brazing encapsulate or laminate for use in joining aluminum surfaces which is capable of being totally consumed or converted during a brazing procedure.
It is another object of the present invention to provide a brazing procedure which does not require the application of brazing flux to the joint area.